Carbon fibers mainly used are divided into PAN-based carbon fibers mainly using acrylic fibers (PAN fibers) as a starting material, and pitch-based carbon fibers using pitches as a starting material. Among them, the PAN-based carbon fibers are utilized mainly in the form of long fibers as it is difficult to obtain spun yarn having high tensile strength in a case of using short fibers, which are only impregnated with a sizing agent and applied to fabrics by use of a high-speed loom. However, despite good performances thereof, those fabrics have a problem of limited applications due to reasons such as high prices thereof.
On the other hand, the pitch-based carbon fibers are divided into anisotropic pitch-based carbon fibers and isotropic pitch-based carbon fibers. The anisotropic pitch-based carbon fibers possess crystal perfection and a highly oriented structure of a hexagonal net plane in the direction of fiber axis that lead to a high elastic modulus and insufficient flexibility, and therefore, the anisotropic pitch-based carbon fibers have a problem of difficulty in weaving with a high-speed loom.
Meanwhile, the isotropic pitch-based carbon fibers are generally manufactured as low-cost and highly productive short fibers, and the short fibers intertwine relatively favorably in a spinning process due to a lower elastic modulus thereof as compared to the anisotropic pitch-based carbon fibers. However, since the single fiber has low tensile strength and is weak against bending or twisting and the number of twist thereof is also fewer than cotton yarn or the like, spun yarn having high tensile strength cannot be obtained.
Therefore, it is difficult to perform weaving with a high-speed loom only by impregnating a sizing agent because a trouble such as breaking spun yarn may be incurred. For this reason, fabrics have been conventionally manufactured with a low-speed shuttle loom by necessity.
Moreover, ends of the short fibers constituting the isotropic pitch-based carbon fiber spun yarn are fluffy. Accordingly, fluff is broken due to friction with guides, rollers, and the like through a spinning or weaving process and is apt to fly apart. Thus, there is a problem of broken carbon fiber dust rising inside a factory as drifting dust that leads to significant deterioration in work environments.
Meanwhile, the conventional isotropic pitch-based carbon fiber spun yarn fabric is obtained by spinning the short fibers and subsequently by weaving the spun yarn with a low-speed shuttle loom. However, spun yarn having a large diameter is used because the number of fibers is generally increased in order to provide the spun yarn with weavable strength. For this reason, the fabric thus obtained has poor flexibility and has a problem of limited applications due to a reason that it is difficult to form a member in a complicated shape.
Under these circumstances, Japanese Unexamined Patent Application Publication No. 2002-54039 (Document 1), for instance, discloses untwisted yarn configured to wind splicing yarn made of cationic dye-dyeable polyester spirally around an outer periphery of a virtually untwisted fiber bundle. The specification (Paragraph 0016) describes that “the filament yarn is dissolved in an alkaline aqueous solution but not in water; therefore, the untwisted yarn of the present invention can employ a water-related process freely in a manufacturing process before completion of weaving.” Nevertheless, methods disclosed in conventional documents like this is not cannot thoroughly resolve the above-described problems of the isotropic pitch-based carbon fiber spun yarn and the fabric thereof.